Aerogel/PTFE Composite Insulating Material

ABSTRACT

A material comprising aerogel particles and a polytetrafluoroethylene (PTFE) binder is formed having a thermal conductivity of less than or equal to 25 mW/m K at atmospheric conditions. The material is moldable or formable, having little or no shedding of filler particles, and may be formed into structures such as tapes or composites, for example, by bonding the material between two outer layers. Advantageously, composites may be flexed, stretched, or bent without significant dusting or loss of insulating properties.

RELATED APPLICATIONS

The present application is a divisional application of U.S. Ser. No.11/039,697, filed Jan. 20, 2005, which is a divisional application ofU.S. Ser. No. 10/706,777, filed Nov. 10, 2003 and now issued as U.S.Pat. No. 7,118,801.

BACKGROUND OF THE INVENTION

The present invention relates to a material that is useful in multipleapplications including insulation applications for garments, containers,pipes, electronic devices and the like. Among other things, the materialof the present invention comprising aerogel particles andpolytetrafluoroethylene (PTFE), is formable, having low particleshedding and low thermal conductivity. Composites made from the materialmay be flexed, stretched, and twisted, with little or no shedding ofaerogel particles or loss of conductive properties.

Use of aerogels for thermal insulation and the low thermal conductivityof aerogels is well known. Favorable thermally conductive propertiesresult from the very high porosity of aerogel which is greater thanabout 95%, and the small pore size of aerogel material which is lessthan the size of the mean free path of air molecules at atmosphericpressure, or less than about 100 nm. Because of the small pore size, themobility of air molecules within the material is restricted, and theeffectiveness of air in conducting heat is reduced, resulting in lowthermal conductivity. Under atmospheric conditions air has a thermalconductivity of about 25 mW/m K (milliwatt per meter Kelvin). Insulationhaving larger pore sizes, such as foam, batting, wool, and other commonthermally insulating materials, has a thermal conductivity of about 40mW/m K, which is higher than that of air due to the contribution ofradiation and solid conduction. Aerogel powders and beads are known tohave a thermal conductivity of about 9 to 20 mW/m K. However, suchhighly porous and low density material is not useful for manyapplications in the form of a powder due to the extensive dusting whichmakes installation, handling, forming and shaping particularlydifficult, and further raises safety issues.

Traditional methods of making aerogels usually include extraction withsupercritical fluids. The methods often include the steps of pouring anaerogel precursor liquid into a mold, drying the aerogel liquid to forma highly porous gel structure with a variety of liquid exchanges, andusing supercritical fluid extraction to form an aerogel monolith.Processes, such as those using supercritical fluid extraction, are verytime consuming and expensive. Further, the structures produced are rigidand have low mechanical strength and have limited ability to be furthermolded or formed into desired shapes after the aerogel material isformed. These materials often crack or shatter upon flexing and areknown for shedding or “dusting” of fine aerogel particles.

In an attempt to increase the flexibility and strength of aerogelmaterial, Stepanian et al. U.S. Patent Publication 2002/0094426 teachaerogel materials combined with a reinforcing structure, specifically alofty fibrous batting. Preferably, the aerogel is reinforced by afibrous batting structure in combination with randomly orientedmicrofibers and/or conductive layers. To form the aerogel sheet, anaerogel-forming precursor liquid is poured into the batting andsupercritically dried to form an aerogel. It is taught that theresulting reinforced aerogel structure is drapable, less prone toshattering upon flexing and less prone to shedding of fine aerogelparticles. However, applications for such materials are limited due to alack of moldability and formability of these structures, as well as thecosts associated with supercritical extraction steps.

To overcome the brittleness often associated with reinforced aerogels,U.S. Pat. No. 5,786,059, to Frank et al. teaches gluing aerogel powderstogether to form a continuous product. Specifically, an aerogelcomposite material having a layer of fiber web and aerogel particles ispreferably formed as a mat or panel. The fiber web comprises abicomponent fiber material of two firmly interconnected polymers havinglower and higher temperature melting regions into which aerogelparticles are sprinkled. Upon heating to the lower melt temperature, thefibers of the web are bonded to each other as well as to the aerogelparticles. The resulting composites are relatively stiff structures, andupon the application of mechanical stress, granules break or becomedetached from the fiber so that aerogel fragments may fall out from theweb.

Smith et al., in U.S. Pat. No. 6,172,120, disclose a process for themanufacture of aerogels wherein aerogels are formed as powders insteadof monolithic blocks or sheets. The manufacturing process has theadvantage of aerogel formation without the step of supercritical fluidextraction. However, in the form of a powder, aerogel is not useful formany applications due to high dusting and lack of formability.

There is a need for an insulating material that overcomes problemsinherent in aerogel powders and composites, such as the lack offormability of aerogel powder and the lack of flexibility of composites,as well as the shedding or dusting of aerogel particles upon applicationof mechanical stress. There is a need for low conductivity, low sheddinginsulating materials that can be molded or formed into any desired shapesubsequent to the formation of the insulating material composition, andwhich, therefore, can be easily suited to a wide variety ofapplications. Further, insulating materials which may be preparedwithout the high processing cost often associated with aerogelinsulation would be greatly desired.

SUMMARY OF THE INVENTION

Formable, moldable, low dusting materials with low thermal conductivityhave been invented and are disclosed herein. These materials aresufficiently moldable to be formed into flexible three-dimensionalstructures or shapes having curves in one or more directions. Further,the materials optionally form stretchable structures with minimaldusting upon stretching.

Materials of the present invention comprise aerogel particles andpolytetrafluoroethylene (PTFE) as a binder. Preferred materials may beformed having a consistency or texture of powder or putty, and also maybe formed into shaped structures having good mechanical strength andflexibility. Properties of the shaped structures, such as mechanicalstrength and thermal conductivity may be affected partly by the relativeproportions of the aerogel and PTFE. For example, higher proportions ofaerogel particles typically result in lower thermal conductivity andhigher proportions of PTFE typically result in a structure with greatermechanical strength.

In use, the material may be contained between multiple surfaces of anarticle, such as the rigid surfaces of a double walled pipe, duct orcontainer to form, for example, an insulated article. Alternately, acomposite structure may be formed by containing the material of thepresent invention between other materials, such as outer layers offlexible membranes, films or foils of polymer, paper or metal.Optionally, elastic membranes are used, making the composite structurestretchable and highly drapable. Likewise, outer layers may be gaspermeable or impermeable to liquids and gases, forming either permeableor impermeable composite structures. Where the composite structure isimpermeable, the thermal conductivity may be further lowered by applyinga vacuum to form a composite structure having reduced pressure.

In addition to use as thermal insulation, it has been found thatcomposites made from the materials of the present invention may also beuseful dielectric materials when constructed with components havingexceptionally low dielectric constants. For example, in one embodimentwhere the dielectric constant of an aerogel is about 1.1 and that ofPTFE is about 2.1, a preferred composite of these materials has adielectric constant of less than about 1.18. Composites made frommaterials of the present invention may also be used as acousticbarriers. Composite material having good sound absorption at specificfrequencies, may be formed by using materials of the present inventionbetween particular outer layers that act as reflective sound barriers.Preferred composites for this application have a relatively high soundtransmission loss of up to about 20 to 30 dB over a range of frequencies(about 400-6300 Hz), and a high sound absorption coefficient of up toabout 0.5-0.6 at a frequency of about 3500 Hz.

Preferred applications for materials of the present invention includethe formation of insulating materials and composites made therefrom foruse in apparel, such as glove and footwear insulation inserts, garments,and inserts for garments, pipe insulation, cryogenic insulation,electronic devices, cookware, home appliances, storage containers andpackaging of food and pharmaceuticals, immersion suits, as well as dualfunction insulation, such as acoustic and thermal insulation, electricand thermal insulation, and the like.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an SEM of an insulating tape material containing 60% wtaerogel and 40% wt PTFE magnified at 10000×.

FIG. 2 is an SEM of an insulating tape material containing 80% wtaerogel and 20% wt PTFE magnified at 10000×.

FIG. 3 is an SEM of insulating powder material containing 80% wt aerogeland 20% wt PTFE magnified at 10000×.

FIG. 4 is a cross-sectional view of a tape having two membrane layerswith insulating material between the layers.

FIG. 5 is an SEM of a tape having two membrane layers with insulatingmaterial containing 60% wt aerogel and 40% wt PTFE between the layersmagnified at 50×.

FIG. 6 a is a diagrammatic half-sectional representation of adouble-walled pipe with insulating material between pipe walls.

FIG. 6 b is a diagrammatic representation of a cross-sectional view of adouble-walled pipe with insulating material between pipe walls.

FIG. 7 is a diagrammatic representation of a perspective view of a pipewrapped with a tape of the present invention.

FIG. 8 a is a diagrammatic representation of an insulating glove insertof the present invention.

FIG. 8 b is a diagrammatic representation of an insulated glove of thepresent invention.

FIG. 9 is a perspective view of an insulated cell phone device of thepresent invention.

FIG. 10 is a perspective view of an insulated portable computer deviceof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The low thermally conductive material of the present invention comprisesaerogel particles and a polytetrafluoroethylene (PTFE) binder.Advantageously, the material may be formed as a powder or putty. By“putty” it is meant that the material has a dough-like consistency thatis formable or moldable, having little or no shedding of fillerparticles. Thus, the material of the present invention has reducedshedding or dusting of fine particles compared to otheraerogel-containing material. The material is useful as a powder orputty, for example, between two surfaces, or as an insulating structurein a molded or shaped form, or as a composite. Preferred compositesinclude insulating structures, such as tape, having the material of thepresent invention bonded between two outer layers, such as films ormembranes. Composites advantageously may be made stretchable, flexibleand bendable, without significant dusting or loss of insulatingproperties.

Aerogel particles are preferred filler materials and are known to haveexceptionally low density and low thermal conductivity. Preferredaerogels have a particle density of less than about 100 kg/m³ and athermal conductivity of less than or equal to about 25 mW/m K atatmospheric conditions (about 298.15 K and 101.3 kPa), and morepreferably, less than or equal to about 15 mW/m K. Aerogels suitable foruse in the material of the present invention include both inorganic andorganic aerogels, and mixtures thereof. Useful inorganic aerogelsinclude those formed from an inorganic oxide of silicon, aluminum,titanium, zirconium, hafnium, yttrium, vanadium, and the like, withsilica aerogels being particularly preferred. Organic aerogels are alsosuitable for use in the present invention and may be prepared fromcarbon, polyacrylates, polystyrene, polyacrylonitriles, polyurethanes,polyimides, polyfurfural alcohol, phenol furfuryl alcohol, melamineformaldehydes, resorcinal formaldehydes, cresol, formaldehyde,polycyanurates, polyacrylamides, epoxides, agar, agarose, and the like.

Aerogels in powdered form are readily available commercially. Forexample, a silica aerogel formed by a relatively low cost process isdescribed by Smith et al. in U.S. Pat. No. 6,172,120. The size ofaerogel particles can be reduced to a desired dimension or grade byjet-milling or other size reduction techniques. For making insulatingmaterials of the present invention, aerogel particles milled to particlesizes ranging from about 5 μm to about 1 mm (aerogel beads) arepreferred, with particles milled to less than or equal to about 10 μmbeing more preferred. It is believed that smaller aerogel particles forma more uniform mix with other components of the insulating material.Aerogels having smaller pore sizes, for example, an average pore size ofless than or equal to about 100 nm are therefore preferred over aerogelshaving larger pore sizes.

Aerogels are available in both hydrophilic and hydrophobic forms.Hydrophilic aerogels typically have a higher thermal conductivity ofabout 18 mW/m K or more, and may be less useful for certain thermaland/or electrical insulation applications due to water absorption.Hydrophobically treated aerogels typically have thermal conductivity ofless than about 15 mW/m K, which is lower than that of hydrophilicaerogels, and have good water repellency, which makes them very suitablefor many thermal insulation applications. Hydrophobic aerogels with athermal conductivity of about 14 mW/m K or lower are preferably used forinsulating material described in this invention. Particle size reductiontechniques, such as milling, may affect some of the external surfacegroups of hydrophobic aerogel particles resulting in partial surfacehydrophilicity, while hydrophobic properties are retained within theparticle. However, aerogel having partial surface hydrophilicity mayexhibit enhanced bonding to other compounds and may be preferred forapplications where bonding is desirable.

The material of the present invention further comprises PTFE. It is mostpreferred that PTFE particles are used that are smaller than the aerogelparticles, for example ranging from about 50 nm to about 600 μm;however, similarly sized PTFE particles are also useful. PTFE primaryparticles having a size of about 50 nm or greater, and PTFE aggregatesof about 600 μm or less are preferred. To form material in accordancewith the present invention, an aerogel/binder mixture is prepared whichpreferably comprises greater than or equal to about 40% wt of aerogel,greater than or equal to about 60% wt, or greater than or equal to about80% wt aerogel. Preferred mixtures comprise an aerogel/PTFE mixturecomprising between about 40% wt to 95% wt aerogel, and 40% to about 80%wt aerogel. PTFE particles comprise preferably less than or equal toabout 60% wt of the aerogel/PTFE binder mixture, less than or equal toabout 40% wt of the mixture, or less than or equal to about 20% wt ofthe aerogel/PTFE binder mixture. Preferred mixtures comprise anaerogel/PTFE mixture comprising between about 5% wt to 60% wt PTFE, and20% wt to about 60% wt PTFE. Properties such as thermal conductivity,dusting, formability and strength may be tailored in part by varying theratio of the weight percentage of aerogel to PTFE in the mixture. Forexample, as the aerogel proportion increases, the strength of structuresformed from this material may diminish and the thermal conductivity maybecome lower. Likewise, material having a higher proportion of PTFE mayprovide structures having higher mechanical strength, greaterformability and minimal dusting.

The material of the present invention may optionally comprise additionalcomponents. Optional components may be added to the aerogel/PTFE bindermixture such as finely dispersed opacifiers to reduce radiative heattransfer and improve thermal performance, and include, for example,carbon black, titanium dioxide, iron oxides, silicon carbide, molybdenumsilicide, manganese oxide, polydialkylsiloxanes wherein the alkyl groupscontain 1 to 4 carbon atoms, and the like. Additionally, polymers, dies,plasticizers, thickeners, various synthetic and natural fibers, areoptionally added, for example, to increase mechanical strength and toachieve properties such as color and thermal stability, elasticity andthe like. Optional components are preferably added at less than about10% wt of the mixture used to form the aerogel/PTFE binder material ofthe present invention.

The material of the present invention may be formed by a number ofmethods including coagulation and dry blending of the aerogel and PTFEcomponents. Coagulation and dry blending of compositions comprising PTFEare known in the art and are described, for example, in U.S. Pat. Nos.4,985,296 and 6,218,000, and in the present examples. One methodparticularly useful in forming the material of the present inventioncomprises the steps of forming a mixture of an aqueous dispersion ofaerogel particles and a PTFE dispersion, and coagulating the mixture byagitation or by the addition of coagulating agents. The resultingco-coagulation of the PTFE in the presence of the aerogel particlescreates an intimate blend of the PTFE and the aerogel particles. Thecoagulum is drained and dried in a convection oven at about 433.15K.Depending on the type of wetting agent used, the dried coagulum may bein the form of loosely bound powder, or in the form of soft cakes thatmay then be chilled and ground to obtain the insulating material in theform of a powder. This powder may be further processed to form, forexample, a putty, shaped structure, or insulated article.

Preferably, the PTFE dispersion is an aqueous colloidal dispersion ofhigh molecular weight PTFE particles formed by emulsion polymerization.PTFE emulsion polymerization methods are known, and methods useful forproducing materials suitable for the present invention are describedextensively in the literature (e.g. S. V. Gangal, “TetrafluoroethylenePolymer”, Mark's Encyclopedia of Polymer Science and Engineering, vol.16, p 577, John Wiley & Sons, 1989.) Suitable aqueous dispersionscomprise about 20% to 70% by weight of PTFE having primary particlesranging in size from about 0.05 μm to about 0.5 μm. Examples of aqueousPTFE dispersions presently commercially available include Teflon™ 30,Teflon™ 35 (from E.I. Dupont de Nemours, Wilmington, Del.), and asFluon™ AD300S, Fluon™ AD704 (from Asahi Fluoropolymer, Chadds Ford,Pa.). Thus, one further embodiment of the present invention is directedto a dispersion comprising an aqueous dispersion of aerogel particlesand an aqueous dispersion of high molecular weight PTFE particles.

Also useful, is a PTFE component comprising fine powder PTFE, obtainedby coagulating PTFE dispersions formed from emulsion polymerization anddrying the flocculated coagulum into a fine powder. Preferred PTFE finepowders typically are of very high molecular weight and are in the formof aggregates of PTFE primary particles, wherein the aggregates range insize from 200 μm to about 600 μm. Preferred PTFE fine powder resins areavailable as Teflon™ 60, Teflon™ 6C, Teflon™ 610A (E.I. Dupont deNemours, Wilmington, Del.), and as Fluon™ CD123 (Asahi Fluoropolymer,Chadds Ford, Pa.). The method of dry blending comprises the processingsteps of dry blending aerogel and PTFE particles, with or withoutlubricants. Processing at different shearing rates and temperatureregimes may result in varying uniformity of the mixture components inthe material of the present invention, as well as differing mechanicalstrengths of the resulting insulating structures. Preferred methods ofdry blending at higher shearing rates and at lower temperature oftenyield material having higher uniformity of components and producestructures having higher mechanical strength.

The material of the present invention may also be subjected to shearstress or force while being molded, formed or otherwise processed intostructures or composites, which may affect the properties of theresulting material. In one embodiment, upon the application of shearstress at about 303.15-453.15 K, the material of the present inventionforms a putty. Though not wishing to be bound by theory, it is believedthat when PTFE primary particles are subjected to deformation whilebeing in contact to one another, PTFE particles become connected by theformation of fine fibrils (S. Mazur, “Paste Extrusion ofPoly(tetrafluoroethylene) Fine Powders” in Polymer Powder Technology, p441, John Wiley & Sons, 1995). It is believed that the size of thesefibrils ranges from about 0.02 μm to about 0.1 μm in diameter, typicallyabout 0.04 μm. It is further believed that shear or extensionaldeformation-induced fibrillation of the PTFE primary particles leads tocreation of a scaffold or cage-like structure. Preferably, the fibrilsof the PTFE particles interconnect with other PTFE fibrils or to PTFEparticles to form a net within and around which the aerogel particlesare situated, and are thus bound together. Therefore, for the materialof the present invention comprising aerogel particles and a PTFE binder,it is thought that because of the unique interconnected microstructureof the PTFE phase, the aerogel has improved binding leading to a numberof beneficial features such as better handleability, improvedmoldability, lower dusting/particle shedding, better flexibility and soon.

Where the material of the present invention comprises aerogel particlesand a PTFE binder, by “binder,” “bind,” or “binding” it is meant thatparticles of aerogel are caused to be held together or cohere with otheraerogel particles, or additional optional components by the PTFEcomponent. Preferably, at least a portion of the PTFE component isfibrillated and at least a portion of the fibrils interconnect,attaching to other PTFE fibrils or to PTFE particles, holding theaerogel particles and binding the aerogel/PTFE structure together. Thus,preferred material of the present invention comprises aerogel particlesand a fibrillated PTFE component comprising interconnected fibrils.

The degree of fibrillation of PTFE primary particles may depend onseveral factors such as the magnitude of shear forces applied,temperature, presence of any lubricating fluid between the primaryparticles, and the like. A higher degree of fibrillation of PTFEparticles is believed to yield material with higher formability, lowerparticle shedding and higher mechanical strength of the resultingstructures or composites. Preferred material of the present invention isnon-dusting or low-dusting, and while the degree of aerogel binding orPTFE interconnection may vary depending upon factors such as amounts ofPTFE and the magnitude of shear forces applied, it is believed thatbinding advantageously reduces aerogel dusting when the insulatingmaterial is molded, flexed, stretched, twisted, wrapped, or otherwisemanipulated. Fibrils extend from the PTFE particles to other PTFEfibrils or particles as depicted in FIGS. 1 and 2. FIG. 1 is an SEM of apreferred insulating tape comprising 60% wt aerogel and 40% wt PTFEmagnified at 10000×. FIG. 2 is an SEM of the preferred insulating tapeof the present invention containing 80% wt aerogel and 20% wt PTFEmagnified at 10000×. FIG. 3 is an SEM of a coagulated aerogel and PTFEpowder mixture containing 80% wt aerogel and 20% wt PTFE magnified at10000×. The tapes of FIGS. 1 and 2, made through the application ofshear or extensional forces, may show higher degree of PTFE fibrillationthan the co-coagulated powder (FIG. 3).

The thermal conductivity of preferred materials of the present inventionhaving about 40% to greater than about 80% wt aerogel, is less than orequal to about 25 mW/m K at atmospheric conditions. These materialspreferably have a density of between about 100-400 kg/m³. Furtherpreferred are materials that have thermal conductivity of less than orequal to about 20 mW/m K, and most preferably, less than or equal toabout 17 mW/m K at atmospheric conditions. In one embodiment, aninsulating material is formed comprising a mixture of hydrophobicaerogel particles and a binder comprising hydrophobic PTFE particles.Insulating material is formed that is highly hydrophobic and waterresistant at atmospheric conditions; such material is therefore suitablefor many thermally insulating applications.

The material of the present invention may occupy a cavity or spacebetween at least two surfaces to form an insulated article. Thesurfaces, independently, may be either flexible or rigid. One embodimentcomprises an insulated article having at least two surfaces, andmaterial of the present invention located between at least a first andsecond surface of the article. Preferably, at least one of the surfacesis a rigid surface, and more preferably, the surfaces are solid andnon-porous. For example, an article having at least two surfaces may bea double-walled container, such as double-walled pipes, cryogenicdewars, manifolds, and the like. An insulated article is thereforeformed where the material may substantially fill the space between twowalls of the double-walled container. FIGS. 6 a and 6 b arehalf-sectional and cross-sectional, respectively, diagrammaticrepresentations of one preferred embodiment of an insulated article ofthe present invention comprising a double-walled pipe having an annularpipe cavity 64, and the material of the present invention 61 within thespace between two pipe wall surfaces 62 and 63. Alternatively, thematerial of the present invention may be contained between flexiblesurfaces, such as rubber, polymers, thin metallized foils, textiles, andthe like, for applications including insulating inserts for use inapparel such as gloves, footwear or for use in garments.

The material of the present invention may be formed as a putty, ormolded or shaped into any desired shape, for example, cylindrical,spherical, rectangular, and the like to form an insulating structure.The material may be molded or formed into a structure by any knownprocess such as roll processing, calendering, compression molding, andpaste extrusion to form two or three-dimensional shapes. The thermalconductivity of preferred insulating structures at atmospheric pressureis less than or equal to about 25 mW/m K, further preferred of less thanor equal to about 20 mW/m K, and most preferably, less than or equal toabout 17 mW/m K. Preferred structures have a density of about 100 to 400kg/m³. A preferred method of forming an insulating structure of thepresent invention comprises providing a material comprising aerogelparticles and a PTFE binder, applying shear force to the material, andforming the material into a shaped, insulating structure, wherein theinsulating structure has a thermal conductivity of less than or equal to25 mW/m K at atmospheric conditions.

One embodiment of the present invention is directed to a shapedinsulating structure comprising the material of the present inventionthat has been formed into a tape. Preferred tapes have a thickness ofgreater than or equal to about 0.5 mm, and preferably between about 0.5mm and 10 mm. A preferred tape forming process is paste extrusion,wherein tape having good mechanical strength properties, and which islow to non-dusting is formed. Methods for paste extruding PTFE are wellknown (“Fine powder processing guide” 202809A(2/91) from DupontPolymers, Wilmington, Del.), and are suitable for use in the presentinvention. In this method, the co-coagulated powder comprising aerogeland PTFE is uniformly mixed with a lubricant to create a lubricatedpowder. Lubricants including hydrocarbon lubricants such as mineralspirits, solvent naptha and the like may be used. Depending upon theaerogel content of the dry powder, the amount of lubricant may rangefrom about 100 to about 400 parts by weight based on 100 parts by weightof the dry powder. The lubricated powder, or lubricated powder in theshape of a pre-form, can be charged in the barrel of a paste extruderand extruded with a ram, and optionally rolled thereafter along theextrusion direction using pressure rolls to form a shaped article. Thelubricant is then removed by drying the article. Another common methodof forming insulating tape structures utilizes two-roll processing astaught in earlier patents such as U.S. Pat. Nos. 2,400,099; 2,593,583;4,153,661 and 4,460,642.

A further embodiment is directed to a composite structure comprising thematerial of the present invention that has been placed between twolayers. The material of the present invention is formed as a corebetween two layers such as membrane, film or foil, or combinationsthereof, and the composition of the two layers may be the same ordifferent, depending upon the application. Individual layers mayoptionally include multi-layered forms such as laminates, coatings andthe like. At least one of the layers may be permeable or impermeable togases or liquids. Preferably, at least one of the layers is formed frommaterials selected from polymers, such as expandedpolytetrafluoroethylene (ePTFE), polyurethane, silicone, polyethylene,polyester and the like, rubber, metal foil such as aluminum or copper,or metallized polymer foil.

Core material can be molded to any desired thickness, and preferably hasa density in the range of between 100 to 400 kg/m³. The compositestructure is preferably formed as tape or compression molded partshaving a thickness of greater than or equal to about 0.5 mm, andpreferably from about 0.5 mm to 50 mm or more, depending on theapplication. Preferably, the aerogel comprises greater than or equal toabout 40% wt, greater than or equal to about 60% wt, or greater than orequal to about 80% wt of the core insulating material. In oneembodiment, the material of the present invention forms a core that isbonded between the two outer layers. In one embodiment core materialbonds to at least one outer layer of a composite, for example byadhering or sticking to surfaces such as films or membranes suitable foruse in the present invention. Composite structures may be formed inwhich the core material is bonded to outer layers without the need foradhesive compounds. However, in addition to non-adhesive bonding,adhesives, such as fluoropolymer, urethane, silicone, epoxy, and othersuitable adhesives may also be used. FIG. 4 is a cross-section of a tapeof the present invention having two outer layers 41 a and 41 b andinsulating material core 42 between the outer layers, wherein the outerlayers may be the same or different. FIG. 5 is an SEM at 50×magnification of a tape having two outer expandedpolytetrafluoroethylene (ePTFE) membrane layers between which is aninsulating material core comprised of 60% wt aerogel and 40% wt PTFE.

One embodiment of the present invention is an insulating composite tapestructure for use in wrapped applications including pipe and containerinsulation applications. The preferred structure comprises a coreinsulating material having an aerogel proportion ranging from about 40to about 80% wt, and optionally, an opacifier, such as carbon black orTiO₂ in an amount of less than about 10% wt, and two layers of expandedpolytetrafluoroethylene (ePTFE) membrane. The core may be bonded betweenthe two layers of ePTFE with or without the use of an adhesive. However,adhesives may preferably be used where the application or materialselection necessitates a stronger bond. A tape insulating structure maybe used to wrap items such as pipes to form insulated pipes for examplefor use in cryogenic, sub-sea oil and gas transportation pipes, and thelike, various containers, long reactors and dewars. Insulating tapes forwrapping applications are highly flexible, and may wrap around anarticle having a diameter as small as about five (5) mm without dustingor cracking, and without any substantial change or increase in thermalconductivity. By “substantial” it is meant a change in thermalproperties of about ±1 to 3 mW/m K. FIG. 7 is a diagrammaticrepresentation of an insulated article of the present inventionillustrating a perspective view of a pipe 72 wrapped with a tape of thepresent invention 71.

A further embodiment is a composite structure in which at least onelayer is impermeable to liquids and/or gases. Impermeable layers may beselected from, for example, membranes such as ePTFE which render thestructure impermeable to liquid water up to pressures of several hundredkPa. Moreover, metal foils or metallized polymer foils may be used toform composites that are resistant to the diffusion of gases. Metalfoils suitable for use in the present invention include aluminum orcopper. Metallized polyester films, for example, may be used as gasimpermeable layers that maintain vacuum pressures of less than 10 kPafor several years. Impermeable composites are useful in applicationsrequiring materials having resistance to toxic and reactive gases,including military uses, industrial applications and the like.

In one embodiment, material of the present invention is a core betweensealed impermeable outer layers, forming a sealed impermeable compositehaving a thermal conductivity that is further lowered by reducing thepressure within the sealed composite, to below atmospheric pressure byapplication of a vacuum. Preferred materials of the present inventionhave about 40 to 95% wt aerogel for use in applications where thecomposites are under a vacuum of about 10⁻⁶ kPa to about 100 kPa.Preferred materials may have a thermal conductivity of less than orequal to about 10 mW/m K, and more preferably from about 4 to about 10mW/m K, under vacuum of approximately 1.5 kPa when measured at ambienttemperature. Moreover, materials of the present invention are furthersuitable for use in cryogenic applications, where under cryogenicconditions of low temperatures and reduced pressure, the thermalconductivity of the material is further reduced. For example atcryogenic temperature of about 77 K, one embodiment of the presentinvention has thermal conductivity of 13.7 mW/m K at atmosphericpressure, and thermal conductivity of about 1 mW/m K at vacuum of1.33×10⁻⁴ kPa. In another embodiment, under small pressure loads of upto about 10 kPa, which is often a pressure level achieved in gloveapplications, the thermal conductivity of a preferred composite may dropby about 1 to 3 mW/m K due to reduction of the size of the larger poresbetween the aerogel and the PTFE particles. Most preferred tapestructures have low compressibility when subjected to a uniaxialpressure of about 10 kPa, exhibiting compressibility of only up to about25%.

In another embodiment a composite is formed wherein at least one of theouter layers is permeable to gases and vapors. Permeable layers mayinclude layers comprising ePTFE or other microporous membranes, woven ornon-woven fabrics, paper, and the like. Vapor permeable compositestructures may be formed from vapor permeable outer layers and aPTFE-aerogel core material that is permeable to the diffusion of vapors.Examples of vapor or gas permeable membranes for use in outer layers ofthe composite include layers comprising ePTFE, silicone, polyurethane,polycarbonate, paper, and other porous and gas permeable membranes, andmay be used in applications where moisture vapor permeability, orbreathability is desired or required, such as ‘breathable’ clothing andfootwear, ‘breathable’ blankets, and other types of ‘breathable’insulation. In some applications such as outdoor clothing breathabilityand liquid water impermeability are required. This can be achieved bythe use of either microporous membranes or continuous polymer films orcoatings of water vapor permeable polymers such as hydrophilicpolyurethanes, polyesters, and the like. A preferred embodiment of thepresent invention is a vapor permeable composite structure comprising acore of PTFE-aerogel material bonded between two outer layers comprisingePTFE membrane.

Further preferred are stretchable composites formed from elasticmembrane layers and core material located there between. Preferredelastomeric layers are thermoplastic elastomers. Further preferred arelayers comprising polyurethane, polyester, polyamide, and copolymersthereof. Preferred stretchable insulating tape is capable of recoveringmost or all of its original dimensions and may be releasably stretchedwithout a substantial change or degradation of thermal properties. By“substantial” it is meant a change in thermal properties of about ±1 to3 mW/m K. Elastic layers suitable for use in the present inventioninclude polyurethanes, silicones, and the like and copolymers thereof.Applications for stretchable insulating composites include apparel,particularly gloves, socks, hats and stretchable clothing, insulativeblankets, immersion suits, insulative gaskets, insulative bandages, andthe like.

Composites having a core of material between outer film or membranelayers may be made by any method known in the art for formingmulti-layer composites such as vacuum formation, continuous rollprocessing, extrusion, lamination, heat compression, adhesive ornon-adhesive bonding and the like, to form an insulating structure. Theouter layers provide strength, durability, elasticity and otherproperties to the tape, depending on the composition.

Advantageously, structures and composite structures, such as sheets ortapes made from materials of the present invention, are suitable forcutting, and may be cut into shapes appropriate to insulate items suchas coolers, canteens, dewars, hands, feet or other body areas, andelectronic devices such as laptop computers, cellular phones and thelike. Insulating structures may be cut to form insulative inserts whichare incorporated into items, for example, by gluing, sewing, or by beingcontained between surfaces of double-walled containers, between multiplelayers or sleeves of materials, such as textile, metal, cardboard, andthe like. Composite tapes are preferred which are non-dusting upon thecut surface. Tape or sheets may be molded into three-dimensional shapesto insulate irregularly shaped objects. For example, insulating insertsor liners for boots or gloves may be constructed, as illustrated, forexample, in FIG. 8 a for gloves. FIG. 8 a is diagrammatic representationof a insulating glove insert 80 of the present invention, which isillustrated as a composite comprising two outer composite layers 81 aand 81 b, and the insulating material of the present invention 82between the layers 81 a and 81 b. Insulated articles of the presentinvention may have one or more than one insert. For example, theinsulating insert may be placed on the palm side surface of a glove andmay be affixed between outer and inner textile layers (FIGS. 8 b, 84 aand 84 b). Alternatively, insulative inserts 80 may be placed on bothupper 83 a and lower 83 b hand surfaces as illustrated in FIG. 8 b. Eachinsert is incorporated into an article and affixed 85 between inner 84 aand outer 84 b textile layers, by any manner known in the art foraffixing textile layers or other insulation, such as by adhesives,sewing, and the like. Insulative inserts 80 may be structures comprisingthe insulating material of the present invention or may be compositeinsulating structures. Likewise, inserts for boots may be located onupper footwear surfaces such as an upper toe cap region, and lower solesurfaces. Inserts may be incorporated into footwear by any manner knownin the art for incorporating footwear layers and insulation, and may beaffixed between inner and outer shoe materials such as textile, leather,plastic and the like, by sewing, adhesives, and the like.

In another embodiment, insulating structures may be useful in portableelectronic devices such as notebook computers, PDA's (personal digitalassistants), cell phones and the like. As the size of the physicalenvelope or enclosure of the device decreases, and/or functionality ofthe device increases, thermal management becomes a challenge. Increasedfunctionality often generates more heat in certain components ofelectronic devices. Reduced enclosure sizes position heat-generatingcomponents in closer proximity to outer enclosures, increasing the easeat which the components conduct heat to outer enclosure surfaces. Inthese cases, outer enclosure surfaces that are in direct contact with auser may become uncomfortably hot. Insulating materials may form usefulbarriers between heat generating components and device enclosures,thereby eliminating or delaying the transfer of heat from certaincomponents to at least a portion of the device enclosure surface that isdesigned to be in direct contact with a user. The temperature of atleast a portion of the device's enclosure surface that is in contactwith a user can be reduced, and the physical comfort of a user can thusbe increased while the device is being used. Alternately, insulatingmaterials may form a barrier between the outer enclosure surface of adevice and a user, thereby eliminating or delaying the transfer of heatbetween a heated device outer surface and a user.

In one example, as illustrated in FIG. 9, a cellular phone isillustrated having at least one insulating structure 91, wherein theinsulating structures 91 may be placed between a printed circuit board92 with heat generating components 95 (such as a power amplifier,multimedia processor, and the like) and a keypad enclosure surface 96 ofthe cellular phone that is often in contact with a user's face (notshown). An insulating structure 91 may also be placed between a printedcircuit board 92 with heat generating components 95 and a back enclosuresurface 94 of the cellular phone that may contact a user's hand (notshown). In another embodiment (not shown), an insulating structure 91may be located external to an enclosure surface 94 or 96, for examplethe insulating structure may be located on at least a portion of anenclosure outer surface that is designed to be in contact with a user'shand. In FIG. 10, a notebook computer is illustrated having aninsulating structure 101, wherein the insulating structure 101 is placedbetween a printed circuit board 102 with heat generating components 105such as a CPU (central processing unit) or a GPU (graphics processingunit) and a bottom enclosure surface 104 of the notebook computer thatis often in contact with the user's lap (not shown). In anotherembodiment (not shown), an insulating structure 101 may be locatedexternal to at least a portion of the bottom enclosure surface 104.Preferably the insulating component is attached to a portion of anenclosure outer surface that is may be in contact with a user's lap.

In another embodiment, a portable electronic device comprises anelectronic device, such as a cell phone or computer, and carrying case,wherein the carrying case comprising an insulating structure.Preferably, the insulating structure may be located between multiplelayers of a carrying case.

Advantageously, insulating structures with low thermal conductivity maybe formed that are sufficiently thin to be incorporated into suchdevices, or external to such devices, without any increase or withoutany significant increase in the overall dimensions of the physicalenclosure. Preferred insulating structures for use in electronic devicescomprise insulating materials of the present invention and are in theform of a tape or sheet, die-cut or otherwise formed to a dimensionalsize to fit readily into unused space in the device. Preferred arecomposites comprising insulating material of the present inventionbonded between two outer layers, such as ePTFE, however insulatingstructures without outer layers may also be useful. Additional materialsmay be incorporated into the electronic device, which work inconjunction with an insulating material, such as highly thermallyconductive materials. Highly thermally conductive materials may bepositioned to intentionally conduct heat away from an enclosure surface,as opposed to blocking or delaying it.

One embodiment of the present invention is directed to a portableelectronic device comprising at least one heat generating component, anenclosure, and at least one insulating structure located between theheat generating component and the enclosure. While the insulatingstructure may be directly adjacent to heat generating components and anenclosure, it is contemplated that additional components, unrelated tothermal insulation, may be positioned between the components of thisarrangement. Further, a method is disclosed for increasing the comfortof a user of a portable electronic device comprising the steps ofproviding a portable electronic device having at least one heatgenerating component and an enclosure having a surface, placing aninsulating structure between the heat generating component and theenclosure, and preventing or delaying the transfer of heat generatedfrom the components to at least a portion of the enclosure surface.Alternatively, the method comprises the steps of placing an insulatingstructure between the enclosure outer surface and the user, andpreventing or delaying the transfer of heat, generated from the at leastone component, to a user.

In general, the properties of the final structures of the aerogel/PTFEmaterials of the present invention advantageously have greaterformability, reduced dusting, increased mechanical strength and lowerthermal conductivity than other insulating materials and theseproperties may depend in part on the proportions of PTFE and aerogel inthe mixture. The thermal conductivity of the material may further dependon the pore size distribution within the material of the presentinvention, the particle size distribution of both aerogel and PTFE, aswell as the compression during molding or tape formation, or the densityof the resulting insulating structure. The presence of pores greaterthan about 100 nm within the insulating material or structures, that ispores that are greater than the mean free path of air molecules, mayresult in an increased thermal conductivity of the structure. Thefollowing examples are illustrative of the range of compositions andproperties that may be attained by a few of the embodiments of thepresent invention, and are not intended to be limiting.

Tests Test 1: Particle Shedding from the Insulating Structure

The number of particles shed from an insulating structure was determinedusing a USP 788 method for particulate matter in injections. A smallsample of insulating material (between about 0.64 cm×0.32 cm and 1.3cm×1.3 cm) was placed in 150-200 cm³ of 50% wt de-ionized water and 50%wt isopropyl alcohol mixture in a beaker immersed in a sonicated waterbath. The particles were extracted by sonication for 2 minutes (Branson2200 ultrasonic bath). The isopropyl alcohol/water mixture was filteredthrough a 0.22 μm Millipore methyl cellulose filter prior to theexperiment. The number of shed particles per unit volume was thenmeasured using a Laser Particle Counting (LPC) technique by HIAC RoycoLiquid Syringe Sampler 3000A with an 8000A counter and an MC200 laser,in the range of 0.2-5 μm at 100 cm³/min. The number of shed particlesper unit volume was determined by subtracting the reading for the numberof particles in a sample containing the insulating material and thenumber of particles in the blank sample (background count). Thebackground count was at least 1000× lower than the sample count for allmeasurements. The number of shed particles per gram of insulatingmaterial was calculated using the formula (Sample Counts/cm³−Blankcounts/cm³)×extraction volume in cm³/area extracted in grams. The numberof shed particles per area of the insulating material was calculatedusing the formula (Sample Counts/cm³−Blank counts/cm³)×extraction volumein cm³/area extracted in cm². Values were calculated from the knowngeometry of the sample and its bulk density. The total number of shedparticles per mass and unit area was calculated by summing the totalnumber of particles shed over the range of 0.2-2.5 μm.

Test 2: Thermal Conductivity Measurement

Thermal conductivity of samples of the present invention was measuredusing a custom-made heat flow meter thermal conductivity tester atatmospheric conditions (about 298 K and 101.3 kPa). The tester consistedof a heated aluminum plate with a heat flow sensor (ModelFR-025-TH44033, Concept Engineering, Old Saybrook, Conn.) and atemperature sensor (thermistor) imbedded in its surface, and a secondaluminum plate maintained at room temperature, also with a temperaturesensor imbedded in its surface.

The temperature of the heated plate was maintained at 309.15 K while thetemperature of the “cold” plate was kept at 298.15 K. The heat flowmeasurement (in W/m² K) was taken between two layers of thin aluminumfoil (approximately 0.02 mm thick), which covered the plates of theconductivity tester. The diameter of the plates was about 10 cm. Thethickness of the samples was measured using a digital micrometer (modelID-F125E, Mitutoyo Corp., Japan) between two thin rigid surfaces. Theheat flow measurement was normally obtained within about two to fiveminutes after the sample was placed in the tester upon reaching a steadystate.

Thermal conductivity was calculated from the measured heat flow and thethickness of the sample according to the formula: k=L/(1/Q−1/Q₀), wherek is thermal conductivity in mW/m K, L is sample thickness in mm, Q isheat flow in W/m² K, and Q₀ is a heat flow with no sample in place(Q₀=100 W/m² K).

EXAMPLES Example 1

A non-dusting material putty was made comprising about 80% wt aerogelparticles and about 20% wt PTFE.

A 4.37% wt dispersion containing about 0.8 kg of hydrophobic, surfacetreated powder of silica aerogel (Nanogel aerogel, grade OJ0008, CabotCorp., Billerica, Mass.), jet-milled to a particle size of about 7 μm,was made in a 25 liter container by adding about 8.75 kg of isopropylalcohol (VWR International Inc., Bridgeport, N.J.) and 8.75 kg ofde-ionized water while agitating at 750 rpm for about 5 minutes. Theagitation speed was then increased to 1500 rpm. About 0.2 kg of PTFEparticles in dispersion, as 0.875 kg of a 23% wt polytetrafluoroethyleneaqueous dispersion (TEFRMS 153, DuPont, Wilmington, Del.), was rapidlypoured into the mixing vessel. Immediately after, about 0.5 kg of 0.4%wt Sedipure surfactant solution (CF803, Tensid-ChemieVertriebsgesellschaft mbH, BASF Group, Koeln, Germany) was poured intothe mix. The total of the solids in the slurry was about 5.1% wt. Theco-coagulation was completed within about 1.5 minutes.

The slurry containing the coagulum was poured onto a perforated trayover a filtering medium to drain the liquid phase over about one hour.The effluent contained about 0.1% wt solids and appeared clear. Thecoagulum was dried at 433.15 K in a convection oven for 48 hours. Thematerial dried into a cake with a thickness of approximately 20 mm. Thecake was chilled to less than about 263.15 K and hand ground using atight circular motion and minimal downward force through a 3.175 mm meshstainless steel screen while maintaining the material at a temperaturebelow about 263.15K. The insulating material was manually compressed toa putty in a circular mold (approximately 76.2 mm in diameter, 6.35 mmthick) to a bulk density of about 181 kg/m³. The density of the puttywas determined volumetrically by dividing the mass of the putty by thevolume of the circular mold.

The thermal conductivity of the molded cake was measured according tothe test for Thermal Conductivity Measurements (Test 2), describedabove. The thermal conductivity of the molded cake was about 13.52 mW/mK.

Example 2

An insulating composite was prepared having a core material comprising80% wt aerogel and 20% wt PTFE within an envelope of ePTFE membrane.

The material of Example 1, was poured into a rectangular mold havingdimensions of 12 cm×13.3 cm×0.8 cm and slightly compressed into a putty.The mold containing the putty was placed between the two layers ofaluminized plastic foil. The mold was removed leaving the molded puttybetween the two layers of the foil. Three sides of the foil around themolded putty were sealed using a hot iron to form an envelope. Theenvelope containing the putty was transferred into a vacuum packingmachine, and the fourth edge of the envelope was sealed under vacuum of1.5 kPa.

The final vacuum packed form was very flexible, did not form any cracksor edges upon flexing, and had an overall thickness of 7.12 mm andthermal conductivity of 9.55 mW/m K when measured substantiallyaccording to the test for Thermal Conductivity Measurements (Test 2)described above.

Example 3

An insulating structure was made from a material comprising about 80% wtaerogel and about 20% wt PTFE.

In a 75.7 liter open mixing vessel, about 29.2 kg of de-ionized waterwas added to about 29.2 kg. of isopropyl alcohol (VWR International,Inc., Bridgeport, N.J.). To this, about 2.68 kg of hydrophobic, surfacetreated silica aerogel (Nanogel aerogel, grade OJ0008 from Cabot Corp.,Billerica, Mass.; jet-milled to a particle size of about 7 μm) wasadded. Using a standard propeller type impeller, the whole mix wasagitated at 1500 rpm for four (4) minutes and then at 1000 rpm foranother four (4) minutes to obtain a uniform slurry. To this slurry,about 0.68 kg of PTFE solids was added in the form of dispersion (TEFRMS153, DuPont, Wilmington, Del.) containing about 26.3% by weight solids.The mix was agitated at 1000 rpm for one (1) minute to coagulate thePTFE in presence of silica aerogel. The coagulum was filtered, dried,chilled and screened into a granular powder as in Example 1.

About 0.82 kg of the above powder was blended with 2.1 kg of Isopar K™liquid (Exxon Corporation, Houston, Tex.). The blended material was ramextruded at a rate of 0.33 m/minute through a die maintained at about322.15 K, after subjecting it to a vacuum of at least 3.3 kPa. The diehad a rectangular opening that measured 178 mm wide and 2 mm high. Theextruded sheet was dried in a convection oven at about 473.15 K toremove the Isopar K hydrocarbon liquid.

The insulating structure was handleable, and non-dusting, and was about2.4 mm thick. Thermal conductivity was measured substantially accordingto the test for Thermal Conductivity Measurements (Test 2) describedabove, and was about 15.1 mW/m K.

The method described in Test 1, Particle Shedding from the InsulatingStructure, was used to determine the number of particles shed from theinsulating structure. The samples used for the experiment hadrectangular shape and dimensions of 1.3 cm×1.3 cm×0.245 cm, and thedensity of 261 kg/m³. The shedding of the sample was compared to twodifferent Aspen aerogel blankets (Aspen Aerogels, Inc., Marlborough,Mass.), Aspen AR3100 (without carbon black) and Aspen AR5200 (containingcarbon black). Aspen AR3100 sample had dimensions of 1.3 cm×1.3 cm×0.64cm and the density of 100 kg/m³. Aspen AR5200 sample had dimensions of0.64 cm×0.32 cm×0.32 cm and the density of 100 kg/m³. The resultspresented in Table 1 are the average of the measurements for the threesamples of insulation, and are reported in total particles per cm² andtotal particles per gram of sample. The results demonstrate lowershedding of particles from the insulating structures of the presentinvention compared with Aspen AR3100 and AR5200. Table 2 provides acomparison based on the ratio of total particles shed by Aspen aerogelblankets to the insulating structure of Example 3.

TABLE 1 Comparison between total number of particles shed from theinsulating structure of Example 3 and Aspen aerogel blankets (AspenAR3100 and Aspen AR5200) total particles/cm² total particles/gInsulating structure of Example 3 1.21E+06 1.90E+07 Aspen AR31002.33E+06 3.64E+07 Aspen AR5200 8.88E+08 2.78E+10

TABLE 2 Ratio of total particles shed of Aspen aerogel blankets to theinsulating structure of Example 3. total particles/cm² total particles/gAspen AR3100/structure of Example 3 1.93 1.92 Aspen AR5200/structure ofExample 3 7.34E+02 1.46E+03

Example 4

An insulating material was prepared comprising about 55% wt aerogel and45% wt PTFE.

A mix of about 65.3 kg of de-ionized water with about 2.22 kg ofisopropyl alcohol (VWR International, Inc., Bridgeport, N.J.) and about0.44 kg of Zonyl FSO™ fluorosurfactant (Dupont Fluoropolymers,Wilmington, Del.) was prepared by agitating the mix in an 75.7 literopen vessel at about 600 rpm for about 30 seconds using a standardpropeller type impeller. About 1.54 kg of silica aerogel (Nanogelaerogel, grade OJ0008, Cabot Corp., Billerica, Mass.), jet-milled to aparticle size of about 7 μm, was gradually added while stirring at about2300 to 2600 rpm. Agitation was continued for about six (6) minutesuntil a uniform slurry of the silica aerogel was obtained. The slurrywas transferred to a 190 liter enclosed vessel and after stirring it atabout 600 rpm for three (3) minutes with a propeller type impeller,about 4.92 kg of PTFE dispersion with a solids content of about 25.8 wt% was added. Agitation was continued at 600 rpm and the PTFEco-coagulated with the silica aerogel. After about three (3) minutes ofagitation, the mix was poured onto perforated trays over a filteringmedium to drain the liquid. The filtered coagulum was dried at about438.15 K for about 24 hours. The dry coagulum was in the form of looselybound powder, and was stored in a freezer at about 253 K.

Example 5

An insulating tape structure was made from a material comprising about55% wt aerogel and about 45% wt PTFE.

The material of Example 4 was blended with Isopar K™ (Exxon Corporation,Houston, Tex.) in a ratio of about 2.5 kg of Isopar K™ liquid to 1 kg ofthe solid material. The blended material was held at about 322.15 K forat least 24 hours. The blended material was ram extruded at 0.25 m/minthrough a 25 mm diameter barrel after subjecting it to a vacuum of atleast about 3.3 kPa. The die used had a rectangular opening that was 25mm wide×0.8 mm high. Both the die and the barrel were maintained atabout 322.15 K.

The extruded insulating tape was 25 mm wide and was dried in aconvection oven at 473 K for 60 minutes. The resulting tape was strong,handleable, bendable, foldable and non-dusting. The insulating tape hada thickness of 1.08 mm. The thermal conductivity of 25 mm wide tapes wasmeasured substantially according to the test for Thermal ConductivityMeasurements (Test 2) described above, except that a first layer of four(4) strips of tape were laid side by side over the test plate followedby another similar layer of four (4) of tapes that were oriented in adirection perpendicular to the first layer. Thermal conductivity wasmeasured to be about 17.9 mW/m K.

Example 6

An insulating structure was made from material comprising about 55% wtaerogel and about 45% wt PTFE.

The material of Example 4 was blended with Isopar K (Exxon Corporation,Houston, Tex.) in a ratio of about 2.5 kg of Isopar K liquid to 1 kg ofthe solid material. The blended material was ram extruded substantiallyas described in Example 3. The blended material was ram extruded atabout 0.25 m/min through a 25 mm diameter barrel after subjecting it toa vacuum of at least about 3.3 kPa. The die used had a rectangularopening that was about 178 mm wide×0.8 mm high. Both the die and thebarrel were maintained at about 322.15 K.

The extruded insulating tape was dried in a convection oven at 473 K for60 minutes. The insulating tape structure had a thickness of about 2.5mm, a density of about 250 kg/m³, and a width of about 148 mm. Thermalconductivity of the tape was about 18.9 mW/m K when measuredsubstantially according to the test for Thermal ConductivityMeasurements (Test 2) described above.

Example 7

An insulating composite tape was made comprising a core materialcomprising about 55% wt aerogel and about 45% wt PTFE, and two outerlayers of ePTFE membrane.

The insulating tape of Example 6 was laminated to two layers of expandedePTFE membranes to create a composite structure using heat and pressure.A layered structure was prepared having a first layer of an ePTFEmembrane (W.L. Gore & Associates, Inc., Elkton, Md.), having thicknessof about 28 μm, mean flow pore size of about 0.21 μm, and mass per unitarea of about 16.9 g/m², upon which a light sprinkling offluorothermoplastic powder (Dyneon™ THV™ 220A, Dyneon LLC, Oakdale,Minn.) was deposited. The insulating tape of Example 6 was placed uponthe first layer, followed by another sprinkling of THV 220A on top ofthe tape, and a second layer of ePTFE membrane. The entire layeredstructure was pressed at about 473.15 K and approximately 300 kPa usinga Carver Press for about three minutes.

The resulting insulating composite structure was about 2.6 mm thick, haddensity of about 250 kg/m³, and had a thermal conductivity of about 18.4mW/m K when measured substantially according to the test for ThermalConductivity Measurements (Test 2) described above.

Example 8

An insulating composite tape was made comprising a core materialcomprising about 55% wt aerogel and about 45% wt PTFE, and two outerlayers of polyurethane membrane.

The insulating tape of Example 6 was laminated to two layers ofthermoplastic polyurethane film to create a composite structure usingheat and pressure. A layered structure was prepared having first andsecond layers of polyurethane (Dureflex® PT1710S, Deerfield Urethanes,Inc., South Deerfield, Mass.) film having a thickness of about 25.4 μmon both sides of the insulating tape. The layered structure was heatlaminated at about 423.15 K and approximately 340 kPa pressure using aCarver press for about 2 minutes to create an insulating compositestructure. The resulting composite structure was about 2.56 mm thick,had density of about 250 kg/m³, and a thermal conductivity of about 17.2mW/m K when measured substantially according to the test for ThermalConductivity Measurements (Test 2) described above.

Example 9

A material was prepared comprising about 75% wt of aerogel and about 25%wt PTFE.

In a 3.6 liter container, about 2.1 kg of de-ionized water and 0.07 kgof isopropyl alcohol (VWR International Inc., Bridgeport, N.J.) weremixed with about 0.021 kg of Zonyl FSO™ fluorosurfactant (DuPontFluoropolymers, Wilmington, Del.). About 0.07 kg of silica aerogel(Nanogel aerogel, grade OJ0008, Cabot Corp., Billerica, Mass.),jet-milled to a particle size of about 7 μm, was gradually added to themix while agitating at 2000 rpm to form a dispersion. Agitation wascontinued for six (6) minutes until a uniform slurry was obtained. Tothis slurry, about 0.0877 kg of PTFE dispersion with solids content ofabout 26.6 wt % was added while agitating at 1500 rpm. Agitation wascontinued at 1500 rpm for about two (2) minutes and the PTFEco-coagulated with the silica aerogel.

The liquid containing the coagulum was poured over porous trays andfiltered. The filtered coagulum was dried at 438.15 K for about 24hours. The dry coagulum was in form of a loosely bound powder.

Example 10

An insulating tape structure was made from a material comprising about75% wt aerogel and about 25% wt PTFE.

The material of Example 9 was blended with Isopar K (Exxon Corporation,Houston, Tex.) in a ratio of about 3.0 kg. Isopar K to 1 kg ofinsulating material. The blended material was held at about 322.15 K forat least 24 hours. The blended material was ram extruded at 0.25 m/minthrough a 25 mm diameter barrel and a die having an opening of about 25mm wide×8 mm high. Both the die and the barrel were maintained at about322.15 K.

The extruded insulating tape structure was about 25 mm wide, and wasdried in an oven at 473 K for about 60 minutes. The resulting tape wasstrong, handleable and non-dusting, and was 1.07 mm thick. The thermalconductivity of the tape was measured substantially according to Example5 and was about 15.2 mW/m K.

Example 11

A material was formed comprising about 55% wt aerogel, 40% wt PTFE, andabout 5% wt carbon black.

In a 3.6 liter container, 2 kg of de-ionized water was mixed with about0.0142 kg of a hydrocarbon surfactant (Tomadol 1-5™, Tomah ProductsInc., Milton, Wis.). About 0.0641 kg of silica aerogel (Nanogel aerogel,grade OJ0008, Cabot Corp., Billerica, Mass.), jet-milled to a particlesize of about 7 μm, was added to form a dispersion. The dispersion wasagitated at 2000 rpm for about three (3) minutes to obtain a uniformslurry. In a separate container, 0.006 kg carbon black (Ketjenblack™EC300J, Akzo Nobel Polymer Chemicals, Chicago, Ill.) was dispersed in0.5 kg de-ionized water by agitating the mix at 1500 rpm for about two(2) minutes. The carbon black slurry was then added to the silicaaerogel slurry and mixed at 1500 rpm for about two (2) minutes. To thisslurry mixture, about 0.1754 kg of PTFE dispersion with solids contentof about 26.6 wt % was added while agitating at 1500 rpm.

After about two (2) minutes of agitation, the liquid containing thecoagulum was filtered, and the coagulum was dried at about 438.15 K forabout 24 hours. The dry coagulum was in form of a free flowing finepowder.

Example 12

An insulating tape structure was made from a material comprising about55% wt aerogel, about 40% wt PTFE, and about 5% wt carbon black.

The material of Example 11 was blended with Isopar K (Exxon Corporation,Houston, Tex.) at a ratio of about 2.79 kg of Isopar K liquid to about 1kg of the insulating material. The blended material was held at about322.15 K for at least 24 hours. The blended material was ram extruded atabout 0.25 m/min through a barrel having a diameter of 25 mm aftersubjecting it to a vacuum of at least about 3.3 kPa. The die used had arectangular opening of 25 mm wide×0.8 mm high. Both the die and thebarrel were maintained at about 322.15 K. The extruded tape was dried ina convection oven at 473 K for about 60 minutes.

The extruded insulating tape was strong, handleable and non-dusting. Thetape was 1.02 mm thick and its density was 210 kg/m³. Thermalconductivity was measured substantially according to Example 5 and wasabout 16.5 mW/m K.

1. A method comprising the steps of providing a portable electronicdevice having at least one heat generating component and an enclosurehaving a surface, and placing an insulating structure between the heatgenerating component and the enclosure, thereby preventing or delayingthe transfer of heat generated from a heat generating component to atleast a portion of the enclosure surface.
 2. The method of claim 1wherein the insulating structure comprises a material comprising aerogelparticles and a polytetrafluoroethylene (PTFE) binder, the materialhaving a thermal conductivity of less than or equal to 25 mW/m K atatmospheric conditions (about 298.15 K and 101.3 kPa).
 3. The method ofclaim 1 wherein at least part of the enclosure surface is designed to bein direct contact with a user.